ESTRO 36 Abstract Book

S478 ESTRO 36 _______________________________________________________________________________________________

Results Patients show no significant variations in OARs doses during the treatment (Table 1).

which may result in under-dosage of target. We investigated the applicability of a decision suppo rt system developed for photon therapy in a proton therapy setting. Material and Methods Twenty-three consecutive NSCLC patients stage 1B to IV treated with adaptive photon therapy were retrospectively planned using intensity modulated proton therapy. The adaptive protocol was based on geometrical measures of target positioning and large anatomical changes as e.g. atelectasis, as observed on daily CBCT scans. Two surveillance CT-scans were acquired during the treatment course. The consequences of anatomical changes were evaluated by recalculation of the proton plans on the surveillance scans. The CTV receiving 95% of the prescribed dose was analyzed. Proton treatment plans were scaled to prescribed doses of 70, 74 or 78Gy, to investigate if full CTV coverage at 95% of 66Gy = 62.7Gy could be maintained by increasing the prescribed dose. Results Fourteen (61%) patients needed adaptations when treated with protons, given that 95% of the CTV must be covered by 95% of the dose. In comparison, no patients needed adaptation when treated with photons using this criterion. Figure 1 shows CTV coverage for all patients. For proton therapy, the adaptive protocol was found to identify patients with large target under-dosage (six patients, group A). Additionally, under-dosage was observed for another eight patients (group B) with non-rigid changes up to 15mm in the positioning of the bones. The median decrease in coverage for all patients was 92.8% [48.1- 100%]. Robust optimization reduces the decrease in target coverage, but does not eliminate the under-dosage, see Fig.2.All patients in group B would be treated sufficiently when prescribing 74Gy with all CTVs receiving 95% of 66Gy. For patients in group A, only two patients would be treated sufficiently with a 78Gy prescription. A geometric decision support protocol as the present is thus mandatory in order to maintain target coverage of the patients in group A. When increasing the prescribed dose, the maximum dose to important normal tissue such as the oesophagus, trachea, bronchi, and heart increases and may thus be the dose limiting factor. Conclusion Large anatomical changes can be corrected for by an adaptive protocol. Non-rigid positioning erro rs are not identified by the geometrical criteria used for photo ns but can be compensated by an increase in the prescribed dose keeping in mind that this requires additional attention to organs at risk. Robust optimisation reduces, but does not eliminate the risk of under-dosage. Daily imaging and treatment adaptation for a high fraction of patients is mandatory in proton therapy for loco-regional lung cancer.

Target coverage analysis shows large differences between Replanned and Non-replanned (p<0.001). The maximum Youden’s index identifies CTV ΔV95= -5% as an optimized threshold level for replanning (sensitivity=87.5%; specificity=100%). TCP analysis shows large variations between Replanned and Non-replanned (p<0.001). ΔV95 and ΔTCP results are summarized in Table 1. ΔV95 comparison between non-adapted and adapted plans shows significant CTV coverage improvements (Figure 1). Conclusion OARs doses were not affected by anatomical changes in all H&N patients studied. On the contrary, there was a significant difference in the effect of anatomical changes for replanned and non-replanned patients, confirmed by radiobiological changes. Therefore, ART reveal great benefits in target coverage for patients that need replanning which can be identified by a threshold dosimetric index. PO-0876 Treatment adaptation is mandatory for intensity modulated proton therapy of advanced lung cancer L. Hoffmann 1 , M. Alber 2 , M. Jensen 3 , M. Holt 3 , D. Møller 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 2 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany 3 Aarhus University Hospital, Department of Radiation Oncology, Aarhus, Denmark Purpose or Objective Large anatomical changes during radiotherapy are seen for a large proportion of lung cancer patients. Precise delivery of proton therapy is highly sensitive to these changes